SMT2: bvnor, bvnand are binary only; add bvxnor

[kroening]

The SMT-LIB2 expressions bvnor and bvnand are not multi-ary.

Furthermore, this adds the (binary) bvxnor expression.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Attention: Patch coverage is 36.17021% with 30 lines in your changes missing coverage. Please review.

Project coverage is 78.56%. Comparing base (fa299c5) to head (99376ab).
Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	36.17%	30 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8508      +/-   ##
===========================================
- Coverage    78.63%   78.56%   -0.07%     
===========================================
  Files         1728     1728              
  Lines       199408   199589     +181     
  Branches     18316    18293      -23     
===========================================
+ Hits        156797   156805       +8     
- Misses       42611    42784     +173     

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